Air Conditioning System
Air Conditioning Refrigerant Cycle Construction
The refrigeration cycle includes the following four processes as the refrigerant changes repeatedly from liquid to gas and back to liquid while circulating.
Evaporation
The refrigerant is changed from a liquid to a gas inside the evaporator. The refrigerant mist that enters the evaporator vaporizes readily. The liquid refrigerant removes the required quantity of heat (latent heat of vaporization) from the air around the evaporator core cooling fins and rapidly vaporizes. Removing the heat cools the air, which is then radiated from the fins and lowers the temperature of the air inside the vehicle.
The refrigerant liquid sent from the expansion valve and the vaporized refrigerant gas are both present inside the evaporator as the liquid is converted to gas.
With this change from liquid to gas, the pressure inside the evaporator must be kept low enough for vaporization to occur at a lower temperature. Because of that, the vaporized refrigerant is sucked into the compressor.
Compression
The refrigerant is compressed by the compressor until it is easily liquefied at normal temperature.
The vaporized refrigerant in the evaporator is sucked into the compressor. This action maintains the refrigerant inside the evaporator at a low pressure so that it can easily vaporize, even at low temperatures close to 0 °C (32 °F).
Also, the refrigerant sucked into the compressor is compressed inside the cylinder to increase the pressure and temperature to values such that the refrigerant can easily liquefy at normal ambient temperatures.
Condensation
The refrigerant inside the condenser is cooled by the outside air and changes from gas to liquid.
The high temperature, high pressure gas coming from the compressor is cooled and liquefied by the condenser with outside air and accumulated in the receiver/drier. The heat radiated to the outside air by the high temperature, high pressure gas in the compressor is called heat of condensation. This is the total quantity of heat (heat of vaporization) the refrigerant removes from the vehicle interior via the evaporator and the work (calculated as the quantity of heat) performed for compression.
Expansion
The expansion valve lowers the pressure of the refrigerant liquid so that it can easily vaporize.
The process of lowering the pressure to encourage vaporization before the liquefied refrigerant is sent to the evaporator is called expansion. In addition, the expansion valve controls the flow rate of the refrigerant liquid while decreasing the pressure.
That is, the quantity of refrigerant liquid vaporized inside the evaporator is determined by the quantity of heat which must be removed at a prescribed vaporization temperature. It is important that the quantity of refrigerant be controlled to exactly the right value.
Compressor
It compressor performs two main functions:
It compresses low-pressure and low-temperature refrigerant vapor from the evaporator into high-pressure and high-temperature refrigerant vapor to the condenser. And it pumps refrigerant and refrigerant oil through the air conditioning system.
This vehicle is equipped with six-cylinder axle plate compressor (1).
The specified amount of the compressor oil is 240cc (8.0 fl. oz.).
The oil used in the HFC-134a system compressor differs from that used in R-12 systems.
Also, compressor oil to be used varies according to the compressor model. Be sure to avoid mixing two or more different types of oil.
If the wrong oil is used, lubrication will be poor and the compressor will seize or malfunction.
The magnetic clutch connector is a waterproof type.
Magnetic Clutch
The compressor is driven by the drive belt from the crank pulley of the engine. If the compressor is activated each time the engine is started, this causes too much load to the engine. The magnetic clutch (3) transmits the power from the engine to the compressor and activates it when the air conditioning is "ON". Also, it cuts off the power from the engine to the compressor when the air conditioning is "OFF". (Magnetic clutch repair procedure can be found in Compressor Overhaul.)
Condenser
The condenser assembly is located in front of the radiator. It provides rapid heat transfer from the refrigerant to the cooling fins.
Also, it functions to cool and liquefy the high-pressure and high-temperature vapor sent from the compressor by the radiator fan or outside air.
A condenser may malfunction in two ways: it may leak, or it may be restricted. A condenser restriction will result in excessive compressor discharge pressure. If a partial restriction is present, the refrigerant expands after passing through the restriction.
Thus, ice or frost may form immediately after the restriction. If air flow through the condenser or radiator is blocked, high discharge pressures will result. During normal condenser operation, the refrigerant outlet line will be slightly cooler than the inlet line.
The vehicle is equipped with the parallel flow type condenser. A larger thermal transmission area on the inner surface of the tube allows the radiant heat to increase and the ventilation resistance to decrease.
The refrigerant line connection has a bolt at the block joint, for easy servicing.
The condenser assembly (6) is located in front of the radiator. It provides rapid heat transfer from the refrigerant to the cooling fins.
Receiver/Drier
The receiver/drier (5) performs four functions;
- As the quantity of refrigerant circulated varies depending on the refrigeration cycle conditions, sufficient refrigerant is stored for the refrigeration cycle to operate smoothly in accordance with fluctuations in the quantity circulated.
- The liquefied refrigerant from the condenser is mixed with refrigerant gas containing air bubbles. If refrigerant containing air bubbles is sent to the expansion valve, the cooling capacity will decrease considerably. Therefore, the liquid and air bubbles are separated and only the liquid is sent to the expansion valve.
- The receiver/drier utilizes a filter and dryer to remove the dirt and water mixed in the cycling refrigerant.
- The sight glass, installed atop the receiver/drier, show the state of the refrigerant.
A receiver/drier may fail due to a restriction inside the body of the unit. A restriction at inlet to the receiver/drier will cause high pressures.
Outlet restrictions will be indicated by low pressure and little or no cooling. An excessively cold receiver/drier outlet may indicate a restriction.
The receiver/drier of this vehicle is made of aluminum with a smaller tank. It has 300 cc refrigerant capacity.
The refrigerant line connection has a bolt at the block joint, for easy servicing.
Pressure Switch
The pressure switch (Dual pressure switch) (4) is installed on the upper part of the receiver/drier, to detect excessively high pressure (high pressure switch) and prevent compressor seizure due to the refrigerant leaking (low pressure switch), so that the compressor is able to turned "ON" or "OFF".
The pressure characteristics of HFC-134a refrigerant differ from those of R-12. Thus, the pressure switch operation for HFC-134a systems has been changed from R-12.
Expansion Valve
This expansion valve (1) is external pressure type and it is installed at the evaporator intake port.
The expansion valve converts the high pressure liquid refrigerant sent from the receiver/drier to a low pressure liquid refrigerant by forcing it through a tiny port before sending it to the evaporator (2).
This type of expansion valve consists of a temperature sensor, diaphragm, ball valve, ball seat, spring adjustment screw, etc.
The temperature sensor contacts the evaporator outlet pipe, and converts changes in temperature to pressure. It then transmits these to the top chamber of the diaphragm.
The refrigerant pressure is transmitted to the diaphragms bottom chamber through the external equalizing pressure tube.
The ball valve is connected to the diaphragm. The opening angle of the expansion valve is determined by the force acting on the diaphragm and the spring pressure.
The expansion valve regulates the flow rate of the refrigerant. Accordingly, when a malfunction occurs to this expansion valve, both discharge and suction pressures get low, resulting in insufficient cooling capacity of the evaporator.
The calibration has been changed to match the characteristics of HFC-134a.
Evaporator
The evaporator cools and dehumidifies the air before the air enters the passenger compartment. High-pressure liquid refrigerant flows through the expansion valve (2) into the low-pressure area of the evaporator. The heat in the air passing through the evaporator core (1) is lost to the cooler surface of the core, thereby cooling the air.
As heat is lost between the air and the evaporator core surface, moisture in the vehicle condenses on the outside surface of the evaporator core and is drained off as water.
When the evaporator malfunctions, the trouble will show up as inadequate supply of cool air. The cause is typically a partially plugged core due to dirt, or a malfunctioning blower motor.
The evaporator core with a laminate louver fin is a single-sided tank type where only one tank is provided under the core.
Electronic Thermostat
The thermostat consists of the thermosensor and thermostat unit (4) which functions electrically to reduce the noises being generated while the system is in operation.
The electronic thermosensor (2) is mounted at the evaporator core (1) outlet and senses the temperature of the cool air from the evaporator. (3) Temperature signals are input to the thermostat unit. This information is compared by the thermo unit and the results in output to operate the A/C Thermostat relay and turn the magnetic clutch "ON" or "OFF" to prevent evaporator freeze-up.
A characteristic of the sensor is that the resistance decreases as the temperature increases and the resistance increases as the temperature decreases.
Refrigerant Line
Restrictions in the refrigerant line will be indicated by:
1. Suction line - A restricted suction line will cause low suction pressure at the compressor, low discharge pressure and little or not cooling.
2. Discharge line - A restriction in the discharge line generally will cause the discharge line to leak.
3. Liquid line - A liquid line restriction will be evidenced by low discharge and suction pressure and insufficient cooling.
Refrigerant flexible hoses that have a low permeability to refrigerant and moisture are used. These low permeability hoses have a special nylon layer on the inside.